1. Field of the Invention
This document relates to an organic light emitting diode display, and more particularly, to an organic light emitting diode display, which can prevent luminance change and color distortion that are caused by an image display pattern or an outdoor environmental condition.
2. Discussion of the Related Art
Recently, there has been developed various flat panel display that can reduce their weight and size which were disadvantages of a cathode ray tube. The flat panel display includes a liquid crystal display (hereinafter, referred to as “LCD”), a field emission display FED, a plasma display panel (hereinafter, referred to as “PDP”), an electroluminescence EL, and the like.
The PDP among them is simple in its structure and fabrication process, thus the PDP is light, thin, short and small and has been paid attention to as a display which is most advantageous in being made large-sized, but there is a big disadvantage in that the luminous efficiency and luminance thereof are low and the power consumption thereof is high. A TFT LCD to which a thin film transistor (hereinafter, referred to as “TFT”) is applied as a switching device is one of the most widely used flat panel display, but has the problems of narrow viewing angle and low response speed because the TFT LCD is a non-light-emitting device. In comparison with this, the electroluminescence device is broadly classified into an inorganic light emitting diode display and an organic light emitting diode display in accordance with a material of a luminous layer thereof. Especially, the organic light emitting diode display uses a self-luminous device which emits light on its own, and has an advantage in that its response speed is fast and its luminous efficiency, luminance and viewing angle are high.
The organic light emitting diode display has an organic light emitting diode OLED, as in FIG. 1. The organic light emitting diode includes an anode electrode, a cathode electrode, and an organic compound layer HIL, HTL, EML, ETL, EIL formed between the two electrodes.
The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL and an electron injection layer EIL.
If drive voltages are applied to the anode electrode and the cathode electrode, holes within the hole injection layer HTL and electrons within the electron transport layer ETL respectively move to the emission layer EML to form excitons. And, as a result, the emission layer EML emits a visible ray.
The organic light emitting diode display includes a plurality of pixels each including an organic light emitting diode which are arranged in a matrix form. The pixels are selected by selectively turning on the TFT, which is an active element, with a scan pulse, and then digital video data is supplied to the selected pixels, thereby controlling the luminance of the pixels in accordance with the gray level of the digital video data. Each of the pixels includes a driving TFT, at least one switching TFT, a storage capacitor, and so on, and the luminance of the pixels is proportional to a driving current flowing in the organic light emitting diode OLED as in the following Equation 1.
                    Ioled        =                              k            2                    ⁢                                    (                              Vgs                -                Vth                            )                        2                                              [                  Equation          ⁢                                          ⁢          1                ]            
Wherein ‘Ioled’ represents a driving current, ‘k’ represents a constant defined by mobility and a parasitic capacitance of the driving TFT, ‘Vgs’ represents a voltage between the gate and source of the driving TFT, and ‘Vth’ represents a threshold voltage of the driving TFT, respectively.
However, such an organic light emitting diode display has the problems that the luminance is different for each of R, G, and B pixels (PB) depending on an image display pattern or an outdoor environmental condition, and this leads to color distortion.
First, luminance change and color distortion caused by an image display pattern will be described below.
An organic light emitting diode display is driven according to a voltage driving type or a current driving type. Especially, an organic light emitting diode display of a voltage driving type exhibits an IR drop due to a driving current Ioled flowing in the organic light emitting diode OLED and a resistance Ra of power supply lines 1 and 2 as shown in FIG. 2. The IR drop changes the voltage between the gate and source of the driving TFT by raising/dropping a potential of the source electrode of the driving TFT and hence. In other words, the IR drop reduces the voltage Vgs between the gate and source of the driving TFT DT by raising (VSS rise) the potential of the source electrode S of the driving TFT DT by ΔV on a panel using an a-Si (amorphous silicon) backplane as shown in FIGS. 3a and 3b, and reduces the voltage Vgs between the gate and source of the driving TFT DT by dropping the potential of the source electrode S of the driving TFT DT by ΔV on a panel using an LTPS (low temperature polysilicon) backplane as shown in FIG. 4. As a result, as can be seen from the above Equation 1, display luminance becomes lower than a desired luminance level according to the reduction of the voltage Vgs between the gate and source.
Due to the IR drop, a luminance difference between a desired luminance level and an actual luminance level is varied according to an image display pattern. That is, the degree of luminance difference becomes larger in a display pattern shown in (B) of FIG. 5 having a relatively large light emitting area than in a display pattern shown in (A) of FIG. 5 having a relatively small light emitting area. This is because, although the resistance Ra of the power supply lines 1 and 2 formed on the panel is constant regardless of the image display pattern, the overall amount of the driving current flowing in the panel increases in proportion to the light emitting area and accordingly the amount of reduction of the voltage Vgs between the gate and source of the driving TFT caused by the IR drop increases. A more significant issue is that when the voltage Vgs between the gate and source of the driving TFT is changed depending on the image display pattern due to the IR drop, color coordinates are distorted. Since the light emitting efficiencies of the R, G, and B organic light emitting diodes are different from each other because of the characteristics of the material, the amount of a driving current for realizing the same gray level is different for each of the R, G, and B pixels. Therefore, each time the image display pattern is changed, the amount of IR drop and the amount of change in the voltage Vgs between the gate and source of the driving TFT becomes different for each of the R, G, and B pixels. As a result, as shown in FIG. 6, change in luminance according to a light emitting area is varied for each of the R, G, and B pixels, and hence color coordinates are distorted, thus causing color distortion.
Next, luminance change and color distortion caused by an outdoor environment condition will be described below.
As shown in FIGS. 3a and 3b, on a panel using an a-Si (amorphous silicon) backplane, owing to device characteristics of the driving TFT DT, the mobility of the driving TFT DT is varied by the effect of an outside temperature or a photocurrent flows in the driving TFT DT by the effect of outside illuminance. In case of FIG. 3a, the driving TFTs DT are designed to have the same characteristics, and therefore luminance difference among the R, G, and B pixels and color distortion that are caused by the variation of mobility and the generation of photocurrent are not that noticeable. However, in case of FIG. 3b, the driving TFTs DT of the R, G, and B pixels are designed to have different characteristics from one another in order to compensate for differences in the characteristics of the R, G, and B organic light emitting diodes with different threshold voltages Vo, and therefore luminance difference among the R, G, and B pixels and color distortion that are caused by the variation of mobility and the generation of photocurrent are very noticeable.